A. Technical Field
The present invention relates to the field of laser illumination systems, and more particularly, to systems, devices and methods of employing pulsed drive currents to drive the lasers and reduce the speckling noises on the projected images.
B. Background of the Invention
Laser light sources used for projection display have shown advantages compared with the conventional light sources, such as ultra-high pressure (UHP) lamps, high-intensity discharge (HID) lamp and xenon lamps. The laser light sources provide wide color expression with narrow spectrum. The color gamut of a light source based on red, green and blue (RGB) lasers can be easily expanded beyond that of NTSC (e.g. 166%), while the color gamut of a lamp projector is less than that of NTSC. Moreover, the laser light sources have longer lifetime, need shorter response time, require no warm-up time, generate less noise, consume less power, and result in higher contrast ratio. In particular, the laser light sources made from semiconductor diodes are desirable for projection displays due to compactness and efficiency. However, to realize a laser illumination system, laser speckling is an issue that needs to be addressed.
Laser speckles arises when the coherent laser light is scattered from a rough display surface and passes the audience's eyes, an optical system having a finite aperture, to form an image. Slight different phases are introduced to the coherent light scattered from different spots on the rough surface. The optical interference of the light shows up as speckles in the image received by the audience. The size of the speckles is related to many factors, such as the light wavelength, the laser beam size, and the distance between the audience and the display surface. Laser speckles are usually quantified by the speckle contrast. Existence of the speckles degrades the image quality, and as one type of noises, the laser speckles should be reduced to a minimum level.
Reduction of speckling noise is equivalent to reduce the contrast of the speckling pattern. More independent speckle patterns need to be introduced and superimposed to average out on the retina/light receiver. Multiple approaches have been taken to achieve this purpose. In the near field of laser sources, the reduction of laser speckle is implemented by using lasers of different wavelengths, a moving diffuser, a moving aperture or aperture averaging. In the far field of laser sources, the reduction of laser speckle is implemented by using vibrating screen, a fiber or fiber bundles with difference lengths, refractive indices, diffractive optical elements or a rotating polygon mirror with two dimensional scanning. However, all these methods involve adding more components to the optical system, and can increase the complexity, size and cost of the display system dramatically.
The laser light sources desirable for projection displays are based on three semiconductor diodes that generate blue, red and green lasers, respectively. FIG. 1 is a block diagram for a typical laser-based projection display. The RGB lasers 102, 104 and 106 are driven by laser drivers 108, and the generated lights are guided by a beam combiner 110 to a scanner 112 which projects an image to a surface 114. The laser drivers 108 and the scanner 112 are both under control of synchronous video data. Light emission power of the lasers 102, 104 and 106 is correlated with the laser current ILASER injected to the diode as in FIG. 2. ITH is a laser threshold current. If the laser current is below ITH, the light emission power is too small to trigger visible light, and the laser current is represented as a biasing current IBIAS. If the laser current is above ITH, the light emission power is roughly linearly proportional to the diode current ILASER, which is represented as a modulation current IMOD. In real time, the modulation current IMOD is required to reach a level of IOP to generate a desirable emission power POP. The modulation current IMOD varies with time to deliver different light emission power POP and thus different image brightness.
In addition to optical system adjustment, an alternative approach to reduce the speckling noise in a projection display is to use a sinusoid current to drive the laser. The laser illumination is terminated every time the laser current approaches a minimum value which is equal to or slightly below the threshold current ITH. The laser wavelength varies slightly every time the laser light is switched back on, and the speckling noise may be improved. However, if the minimum value is above ITH, speckle reduction is not significant, and if the minimum value is far below ITH, the laser can be damaged due to regular powering on and off.
FIG. 3A illustrates a laser driver 300 with radio-frequency (RF) modulation to reduce speckles and FIG. 3B illustrates an exemplary current waveform 350. The laser driver 300 comprises three current sources 304, 306 and 308. The first current source 304 produces a modulation current IMOD above the threshold current ITH to deliver the desirable emission power POP. The second current source 306 produces an offset current IBIAS below the threshold current ITH. The third current source 308 is particularly used to reduce the speckling noises by generating a high-frequency sinusoid laser current IRF. The modulation and biasing currents IMOD and IBIAS defines an envelope specifying the peaks and valleys of the sinusoid laser current IRF. The frequency of the modulation current IMOD is lower than that of the laser current IRF. The magnitude of IRF is constantly adjusted according to IMOD in order to avoid levels far above and below ITH. Due to the fixed equivalent duty cycle of 50%, the peak laser current (the modulation current) is twice of the target average current, and the emission power and the perceived image brightness are determined by the modulation current.